One example of the hybrid vehicle equipped with a so called “mechanical distribution type drive unit” is disclosed in Japanese Patent Laid-Open No. 2002-225578, and a construction thereof will be described briefly hereafter. In the disclosed hybrid vehicle, an engine torque is inputted to a carrier of a single pinion type planetary gear mechanism which constitutes a distributing mechanism, a first motor generator is connected to a sun gear, and an output member such as a counter drive gear etc. is connected to a ring gear. A second motor generator is connected to the output member or the ring gear through a transmission. The downshift when a clamping pressure for the belt is insufficient due to a shortage of the oil pressure or the like.
On the other hand, in Japanese Patent Laid-Open No. 2000-230442, there is disclosed an invention, which is constructed to inhibit halt of the engine and to establish an oil pressure by a mechanical oil pump in a hybrid vehicle having the mechanical oil pump and an electric oil pump, in case a cumulative load obtained on the basis of a cumulative operating time, a cumulative number of revolution and so on of the electric oil pump exceeds a durable load. In addition, in Japanese Patent Laid-Open No. 2001-112114, there is disclosed an invention, which is constructed to restrict an operation of an electric motor, in case an operating state of the electric motor exceeds a preset heat rating.
As described above, a hybrid vehicle comprises a plurality of prime movers such as an engine and an electric motor, and is capable of running using not only both of those prime movers, but also using any one of those prime movers. Accordingly, in order to ensure the oil pressure when the engine is halted, it is conceivable to provide an electric hydraulic pump which is capable of generating the oil pressure even when the engine is halted, in addition to a hydraulic pump which is driven by the engine. In the construction disclosed in the aforementioned Japanese Patent Laid-Open No. 2002-225578, the transmission is arranged between the second motor generator and the output member. In this construction, in case of running the vehicle by using the second motor generator as the prime mover, the mechanical oil pump is halted in accordance with a cessation of the engine. Therefore, it is necessary to feed the oil pressure to the transmission by driving the electric hydraulic pump so as to put the transmission into a torque transmittable state. In this state, it is required that the oil pressure corresponding to the torque which can maintain the running of the vehicle by the second motor generator, so that it is sufficient for the electric hydraulic pump to have such capacity. As a result of this, the entire construction of the hybrid vehicle can be downsized.
However, according to the construction disclosed in Japanese Patent Laid-Open No. 2002-225578, in case the engine is started due to increase of a demand torque and so on, the first motor generator is operated as the motor so as to carry out a motoring (i.e., or cranking) of the engine by using the motive power, but the torque of the first motor generator is transmitted to the engine through the distributing mechanism composed of the planetary gear mechanism, so that the torque acts on the output member in the direction to rotate the output member backward. Accordingly, it is desirable to output the torque further from the second motor generator at the starting time of the engine, so as to countervail the torque acting on the output member as a result of the motoring by the first motor generator. The torque outputted from the second motor generator at the motoring time is to be added to the torque of the second motor generator to run the vehicle. Therefore, the torque larger than that in the case of running the vehicle by the second motor generator acts on the transmission. It is necessary to raise the oil pressure in order to enable the transmission to transmit the torque sufficiently, even under such condition. However, if the oil pressure is outputted only by the electric hydraulic pump, there arise an disadvantage in that the electric hydraulic pump grows in size. Moreover, since an electric power is supplied from a power source such as the battery to both the first motor generator for the motoring and the electric hydraulic pump simultaneously, there is another disadvantage in that a load on an electric power source increases.
On the other hand, the oil pressure can be established sufficiently when the mechanical oil pump is driven by the engine, so that it is unnecessary to drive the electric hydraulic pump. Therefore, according to the invention disclosed in Japanese Patent Laid-Open No. 2001-41067, in which the electric pump is driven prior to halt the engine, if the electric pump is driven when the mechanical pump is driven by the engine and generating sufficient oil pressure, the oil pressure is generated excessively, and may cause a power loss. Moreover, in case the electric hydraulic pump is provided as a supplemental means to the mechanical oil pump driven by the engine, it is desirable for the electric hydraulic pump to have a required minimum capacity so as to minimize its size. However, the oil pressure is required in various situations, and there may be a case in which the oil pressure higher than the permissible limit of the electric hydraulic pump is required. Therefore, there is room for developing an effective means for determining the permissible limit of the electric hydraulic pump or controlling when the permissible limit is determined.